Definition
Plain language
A keyword in the Lean proof language that lets you skip proving a step by just declaring it true.
As stated in the literature
A Lean placeholder term that unsoundly discharges any proof obligation; a single one buried in a dependency silently invalidates every declaration built on top of it, motivating full dependency-graph auditing.
Why it matters: A single hidden one of these can silently invalidate everything built on top of it, which is why proof libraries must be audited all the way down.
For example, a proof can be made to 'pass' by dropping in this keyword where a hard step should be, declaring it done without actually proving it.
Heard on the show
“Like a professional interpreter — you stammer, you apologize, you say "sorry, this is probably a stupid question.”Episode 241 — Swapping the Name Did Nothing, But Hedging Moved Every Model